Keep your filter between the Min and Max marks. Adjust your focal length and position. Another safe bet is to frame your image so that you can crop the dark corners out later.
Darkened corners is known as vignetting and usually occurs with wide-angle lenses. It’s caused by the wide-angle lens’ wide field of view capturing the edge of the lens filter, lens hood or even the lens rim. To avoid vignetting, try a longer lens, or crop the vignetting out when it comes time to publish or print your image. We also recommend you avoid stacking filters to help reduce vignetting. Even taking off your UV filter when using an ND filter will help.